Patients with hereditary C4 deficiency are likely to have severe lupus erythematosus. A patient with hereditary angioneurotic edema (HANE) and systemic lupus erythematosus (SLE) had a chronic deficiency in C4 because the hereditary deficiency in C1-inhibitor allowed the C1 in her serum to become activated and then inactivate C4. An attempt was made to repair the C4 deficiency as well as the deficiency in C1-inhibitor by giving infusions of human C1-inhibitor in the hope of inducing remissions of both HANE and SLE. During treatment, antibody to C1-inhibitor developed in the patient; this cleared when the infusions were stopped. During subsequent treatment with danazol alone, measurable C1-inhibitor developed in the patient's serum, but levels of C4 were never significantly increased. Antibody to normal C1-inhibitor was not expected to develop in the patient because she is heterozygous for this autosomal dominant trait. A normal allotype (VAL or MET 458), which would have been in the preparation used but which the patient does not synthesize because she can produce only one allotype (MET 458), appears to have been immunogenic. The antibody isolated from the patient's serum reacted with C1-inhibitor from a normal individual known to be homozygous for 458-VAL, but not with one from a homozygote for MET 458.